Ductwork for corrosive vapor exhaust systems is used extensively in many diverse industries which utilize chemicals to process raw materials or perform manufacturing procedures such as the semiconductor industry, the plating industry, the pharmaceutical industry and numerous other industries. Waste water treatment plants also use corrosive chemicals such as chlorine, and caustics, such as sodium hydroxide or sodium hypochlorite in processing sewage, as well as other chemicals. Many research and development labs, and college science buildings also use a great variety of chemicals in conducting experiments in varied fields such as biochemistry, genetics, geological uses, or general chemistry and physics. These chemicals are used for various types of processes or tools, many of which expose personnel in the work environment to hazardous materials. For worker safety, the vapors from these chemicals must be exhausted through air ducts to remove potential contaminants from the work place. Some duct installations can be very large, consisting of many thousands of feet of ductwork which may be manifolded and connected to multiple exhaust fans. Because of the wide diversity of the chemicals used, for various industries, it is exceedingly difficult to provide a single material for ductwork construction which can handle all the chemical exposures which the ducts may face. A great variety of materials have been used heretofore to fabricate fume exhaust ductwork, such as black steel, galvanized steel or stainless steel as well as plastic materials such as polyvinylchloride, polypropylene, coated metals and fiberglass reinforced plastic. In the last forty years the trend in the use of materials has been away from metals or coated metals and toward the use of plastics, the most popular being FRP (fiberglass reinforced plastics).
In the fiberglass fabrication industry, various types of resins have been used over the last three decades. Amongst them are bisphenol fumarates, epoxies, chlorendic anhydrides, isopthalic or orthopthalic resins, and vinylester resins. A problem common to all plastics has been flammability because they can burn rapidly and produce much smoke, creating hazards of their own. Efforts have been made to reduce the flammability of the material by incorporation of various chemical mechanisms such as antimony oxides, boron compounds and heat absorbing fillers, e.g. aluminum trihydrate. Hybrid resins which eliminate the use of styrene have been attempted, such as methyl methacrylate, and various mixes of resins have been tried. The plastics industry often refers to certain classes of materials as "fire-retardant". Commonly these incorporate fillers, heat sinks, such as aluminum trihydrate, and most commonly, halogenated resin systems which complex with antimony or boron compounds. The latter function as free radical traps, thereby depriving the surface fuel of oxygen, and interfering with combustion.
Against this background in the 1970's, resin systems and fume exhaust plastic ducts were developed having excellent fire and smoke properties. Such systems are described in my U.S. Pat. Nos. 4,053,447; 4,076,873; 4,107,127. In general, phenols and similar ring structured molecules are recognized as having excellent fire resistance characteristics and they also generate low quantities of smoke. Phenols, in and of themselves, generally require heat and/or pressure in order to effect their cure. An FRP laminate can be developed with the use of phenolic resins alone. Resorcinol, belonging to the phenol family, reduced or eliminated the necessity for the use of heat and pressure to make ductwork. Various formulations afford cure at ambient (room) temperatures.
As described generally in the above patents, various types of aldehydes used in conjunction with resorcinol, or phenol/resorcinol combinations can be used to effect cure of the resin. An excess of aldehydes to the hydroxyl radicals contained within the mix is necessary. Paraformaldehyde, furfuraldehyde, or other aldehydes can be used alone or in combination with various types of phenol/resorcinol mixes.
In addition to the problem of providing an adequate fire resistant duct material a further and increasingly severe problem arose in producing a duct which is also capable of resisting broad classes of air-borne chemicals.
No one duct material can resist all chemicals used in a wide variety of industries. Certain classes of resins have enhanced properties and resistance to certain families of chemicals. For example, polyesters generally have good resistance to acids and to some degree, on caustics; they generally do not have good resistance to solvents, particularly halocarbons. Epoxies generally have good resistance to caustics and solvents, but do not have the best resistance to strong mineral acids. Other materials such as polyvinylchloride, polypropylene, and other materials exhibit the same characteristics, i.e. good resistance to some chemicals, poor resistance to others.
The same is equally true for various combinations of phenol/aldehyde resins. They have good resistance to most acids, but not to such as concentrated sulfuric acid when used with oxidizers such as hydrogen peroxide. These same resin systems have poor resistance to liquid caustics. In those fume exhaust systems handling these types of materials, the PRF resin systems do not provide the chemical resistance available from alternative resin systems.
Attempting to provide the ideal product is difficult because resin systems with enhanced chemical resistance often have very poor fire retardant performance. On the other hand, those resins with enhanced fire retardant characteristics can have poor resistance to certain types of chemicals. Accordingly, a primary object of the present invention is to solve the aforesaid problem and provide a fume exhaust duct that has both good chemical resistance and good fire resistance characteristics.
Another object of the invention is to provide a method for manufacturing a fume duct which has both good chemical resistance and good fire resistance characteristics.